🔍 Implement basic event visibility

See https://framagit.org/framasoft/mobilizon/wikis/spec/Event#visibility

Also brings support for event status (tentative/confirmed/cancelled)

Closes #56

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

test/support/factory.ex

@@ -105,7 +105,7 @@ defmodule Mobilizon.Factory do
       organizer_actor: actor,
       category: build(:category),
       physical_address: build(:address),
-      public: true,
+      visibility: :public,
       url: "@#{actor.url}/#{Ecto.UUID.generate()}"
     }
   end